#15347: QuantifiedConstraints: Implication constraints with type families don't work -------------------------------------+------------------------------------- Reporter: aaronvargo | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 8.5 checker) | Keywords: Resolution: | QuantifiedConstraints Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by aaronvargo): Replying to [comment:16 simonpj]:
Really? What wrong with this?
{{{ Not in scope: type variable ‘ft’ }}} The superclass isn't allowed to mention variables which don't appear in the head, even if they're already determined by other parameters. The solution to this limitation has been "use type families", which leads us to this issue. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15347#comment:17 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler